1. Field of the Invention
The present invention relates to high-voltage transformers.
2. Description of the Related Art
In general, copy machines and printers use high-voltage power supplies. Such devices include a high-voltage transformer with a high-voltage output on the order of DC 10 KV.
For example, Japanese Patent No. 3182799 discloses a high-voltage transformer of this type. This high-voltage transformer includes a transformer section and a voltage multiplier-rectifier circuit section. FIG. 4A is a sectional view showing bobbins of the transformer section. FIG. 4B is a circuit diagram showing pulses generated by the high-voltage transformer. Referring to FIG. 4A, a primary-coil bobbin 21 has a hole for holding a core in its center and two flanges arrayed along the central axis. These two flanges form a winding groove 22. One of the flanges has two terminals 23. A concentric secondary-coil bobbin 24 surrounds the periphery of the primary-coil bobbin 21. The secondary-coil bobbin 24 has five flanges arrayed along the central axis. These five flanges form four winding grooves 25. The outermost flanges each have a terminal 26. Both bobbins have nearly the same length. The transformer section includes the primary-coil bobbin 21, the secondary-coil bobbin 24, a primary coil, a secondary coil, and the core.
Referring to FIG. 4B, a high-voltage transformer 30 has a voltage multiplier-rectifier circuit section, which is a Cockcroft-Walton circuit including diodes D21 and D22 and capacitors C21 and C22. This voltage multiplier-rectifier circuit section is connected to both ends of a secondary coil S21. The cathode side of the diode D21 is a high-voltage output. In this high-voltage transformer 30, a primary coil P21 is connected to a drive circuit (not shown in the drawing) that includes a switching element and operates at a predetermined frequency. This drive circuit detects the output voltage and current and turns the switching element on/off to operate with constant voltage and current.
A positive pulse W21 is generated in the primary coil P21 by the drive circuit, while a positive pulse W22 is generated in the second coil S21. The amplitude of the positive pulse W22 depends on the turn ratio of the high-voltage transformer 30. The voltage multiplier-rectifier circuit section rectifies and multiplies this positive pulse W22 to provide a high-voltage output.
Miniaturization of copy machines and printers, which require a more compact, less expensive high-voltage transformer, has recently been advancing. Conventional high-voltage transformers unfortunately have a large number of components, making it difficult to miniaturize them to reduce their cost. The present inventors examined a single bobbin holding a primary coil and a secondary coil. This bobbin resulted in a loose connection between the two coils. In addition, in this bobbin, the secondary coil generates a pulse having the same amplitude as in a conventional bobbin, thus failing to reduce the distributed capacitance. As a result, the bobbin exhibits poor high-voltage output characteristics, such as load characteristics and frequency characteristics.